[syslinux] lpxelinux.0 problems with CentOS 7.6

[Jason Mann]

Hello list.

This issue seems like it may be related to one described in a post in
May: https://www.syslinux.org/archives/2019-May/026424.html.  I've
only just subscribed, so I was unable to reply directly to that
thread.

I am having a similar issue when I attempt to PXE boot CentOS 7.6.1810
using lpxelinux.0.  The files are fully downloaded from the server and
the kernel starts booting but panics when it attempts to mount the
init ramdisk as its root filesystem.  This occurs when using either
HTTP or TFTP to retrieve the files.

If I switch to pxelinux.0 the kernel boots successfully.

I tested a few other versions with lpxelinux.0 and HTTP:
CentOS 7.5.1804: success
CentOS 7.4.1708: success
CentOS 6.10: success

I am using the official pre-compiled binaries from the syslinux 6.03
tarball downloaded directly from kernel.org, with this configuration:

LABEL centos7-ks
  MENU LABEL CentOS 7.x kickstart
  KERNEL http://filer/repo/centos/7/os/x86_64/images/pxeboot/vmlinuz
  INITRD http://filer/repo/centos/7/os/x86_64/images/pxeboot/initrd.img
  APPEND ip=dhcp inst.ks=http://filer/repo/kickstart/centos7-ks.cfg inst.gpt

In the older thread, there was some talk of the increasing size of the
initrd.img file possibly being an issue, but in my testing, the 7.5
initrd.img is larger than 7.6's but still boots fine

# unxz -l /pool2/repo/centos/7.?.????/os/x86_64/images/pxeboot/initrd.img
Strms  Blocks   Compressed Uncompressed  Ratio  Check   Filename
    1       1     46.2 MiB    130.1 MiB  0.355  CRC32
/pool2/repo/centos/7.4.1708/os/x86_64/images/pxeboot/initrd.img
    1       1     50.4 MiB    133.6 MiB  0.377  CRC32
/pool2/repo/centos/7.5.1804/os/x86_64/images/pxeboot/initrd.img
    1       1     50.1 MiB    132.7 MiB  0.378  CRC32
/pool2/repo/centos/7.6.1810/os/x86_64/images/pxeboot/initrd.img

The fact that the older versions boot fine suggests that something may
have changed on the CentOS side, but conversely, the fact that 7.6
fails to boot with lpxelinux.0 but boots fine with pxelinux.0 suggests
an issue in lpxelinux.0.  I don't know much about these things, but I
would assume that pxelinux.0 and lpxelinux.0 load the vmlinuz and
initrd.img files into memory and execute the kernel in the same way?

What can I do to further diagnose this issue?

JM